Instruction:
Question #1. What is air mass? Discuss conditions for growth of an airmass. How air masses are responsible for modification of weather conditions?
Question #2. Explain is Tri-meridional circulation. Give climatic significances of meridional circulations.
(Examiner will pay special attention to the candidate's grasp of his/her material, its relevance to the subject chosen, and to his/ her ability to think constructively and to present his/her ideas concisely, logically and effectively).
Question #1. What is air mass? Discuss conditions for growth of an airmass. How air masses are responsible for modification of weather conditions?
Air mass is an immense body of air with relative homogenous physical properties in terms of temperature, humidity and density. It develops in horizontal layers produced by thermodynamics modification in the planetary winds. Air mass is part of secondary circulation of wind. They are identified by their contrast physical properties in relation to the surrounding air.
Growth of an airmass:
Airmasses have been classified on the basis of their source region and they have been modified due to thermal condition and dynamic effects.
When an airmasses approaches to destination, there it has been identified by its physical properties on the basis of either temperature of surface or contrast airmass. Consequently, modification in airmass and weather starts occurring.
Modification of weather conditions
Winter time Polar – Continental airmass develops over Greenland, Northern Canada, Siberia and Antarctica. During winters these regions are extremely cold. Therefore, cold airmass develops over these cold surfaces and airmass has been identified as CPKs airmass i.e. continental-polar-cold-stable airmass.
After developing a mature airmass, it comes under effect of polar easterlies and it has been dragged towards warm oceanic surface. Due to heating from below, convection process starts. It results in formation of cumulo-nimbus clouds with thundering and lightening and heavy rainfall occurs but intensity of rainfall depends upon relative humidity in the airmass.
Thus, mixing of CPK becomes unstable and weather disturbances occur.
Similarly, Continental-Tropical-Warm-Stable airmass develops over continental masses in tropical areas. Over a homogenous surface it is a stable airmass as long as it stays over continental airmass. Thus, this airmass has been expressed as CTWs airmass.
CTWs airmass comes under effect of tropical easterly winds and they have been dragged to equatorial zone. It comes in effects strong convection in equatorial zone and it becomes unstable. Here, this airmass has been expressed as CTWu.
In process of convection, it becomes part of vertical ascending air current which produces dense, vertical clouds known as cumulo-nimbus clouds. It precipitates very heavily which is known as torrential rain. Such rain creates extreme weather disturbances with strong thundering, lightening and convectional rain during noon time which is known as 2 pm to 4 pm rain.
It can be concluded as, an airmass confined to stable airmass in initial stage of formation but it certainly creates extreme weather disturbances over different latitudinal zones over the globe.
Question #2. Explain is Tri-meridional circulation. Give climatic significances of meridional circulations.
Tri-meridional circulation is movement of wind vertically and horizontally in between pressure belts. The concept of tri-meridional circulation was put forward by Palmen in 1951.
Tri-meridional circulation as atmospheric circulation form cells of wind flow. These cells are:
Hadley Cell: this atmospheric circulation take place in tropical region therefore, also called tropical cell. It was discovered by Hadley in 1735, thus, also known as Hadley cell.
Ferrel Cell: it is responsible for formation of front in sub-polar region, therefore also called polar front cell. Winds blowing from sub-tropical high pressure zone to sub-polar zone are part of this system i.e. known as westerlies (fig. 1).
Polar Cell: this cell develops between 60 degree to 90 degree latitudes in both hemispheres. It is anticyclonic conditions over sub-polar and polar region due to subsidence of air from atmosphere.
Climatic Significances
It can be concluded in last that tri-meridional circulation is continuous prevailing air flow which affects atmosphere in different facets at short term in form of weather and long term in developing seasons and climate.
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